A field effect transistor (FET) is a three-terminal device having a gate electrode, a source electrode, and a drain electrode. Further, the field effect transistor is an electronic active device in which a current flowing through a channel layer (a current between a source electrode and a drain electrode) is controlled by voltage applied to a gate electrode. In particular, an FET which uses as a channel layer a thin film is called a thin film FET (thin film transistor, TFT). The device can be formed on an several substrates formed of ceramic, glass, plastic, or the like.
The above-mentioned TFT has an advantage of being easily formed on a substrate having a relatively large area because the TFT uses thin film technology, and is widely used as a driving device in a flat panel display device such as a liquid crystal display device. More specifically, in an active liquid crystal display (ALCD), TFTs formed on a glass substrate are used to turn on and off individual image pixels. Also, in a prospective high performance organic LED display (OLED), it is thought to be effective to current-drive the pixels by TFTs. Further, a higher performance liquid crystal display device is realized in which a TFT circuit having a function of driving and controlling the whole image is formed on a substrate in the vicinity of an image display region.
A TFT which is currently most widely used is a metal-insulator-semiconductor field effect transistor (MIS-FET) device using a polycrystalline silicon film or an amorphous silicon film as a channel layer material. An amorphous silicon TFT is practically used for pixel driving and a high-performance polycrystalline silicon TFT is practically used for drive control of the entire image.
However, it is difficult to form the amorphous silicon TFT and the polycrystalline silicon TFT on a substrate such as a plastic plate or a plastic film because a high-temperature process is essential for device formation.
In recent years, active development has been conducted for realizing a flexible display by using TFTs formed on a substrate such as a polymer plate or a polymer film for a driver circuit of an LCD or OLED. Attention is focused on an organic semiconductor film which is made of a material capable of being formed on a plastic film or the like, which can be formed at a low temperature.
For example, pentacene has been under research and development for the organic semiconductor film. The organic semiconductor has an aromatic ring. Large carrier mobility is obtained in the stack direction of the aromatic ring at the time of crystallization. For example, it is reported that, in the case where the pentacene is used for an active layer, the carrier mobility is approximately 0.5 cm2 (Vs)−1, which is equal to that of an amorphous Si-MOSFET.
However, the organic semiconductor such as the pentacene has low thermal stability (it is unstable when temperature exceeds 150° C.) and toxicity (carcinogenicity). Therefore, a practical device has not been realized.
Attention has been recently focused on an oxide material as a material which can be applied to the channel layer of a TFT.
For example, a TFT using, as the channel layer thereof, a transparent conductive oxide polycrystalline thin film containing ZnO as a main ingredient has been under active development. The thin film can be formed at a relatively low temperature, and therefore it is possible to form the film on a substrate such as a plastic plate or a plastic film. However, in the case of a compound containing ZnO as a main ingredient, a stable amorphous phase cannot be formed at a room temperature and a polycrystalline phase is formed. Therefore, it is difficult to increase an electron mobility thereof because of scattering of polycrystalline grain boundaries. In addition, it is difficult to achieve reproductivity in the characteristics of a TFT device because a polycrystalline grain shape and an interconnection thereof are significantly changed depending on film formation methods.
A thin film transistor using an amorphous In—Ga—Zn—O-based oxide has been reported by K. Nomura, et al., Nature, 432, 488 (2004). The thin film transistor can be formed on a plastic plate or a glass substrate at a room temperature. The device shows a normally-off characteristics with field effect mobility of approximately 6 to 9. There is also an advantage in that the thin film transistor is transparent to visible light.
The inventors of the present invention have studied a thin film transistor using an oxide including an amorphous In—Ga—Zn—O oxide. As a result, there is the case where a variation in transistor characteristic (Id-Vg characteristic) of a TFT occurs, although an extent of the variation is depended on the channel materials or the manufacturing conditions etc.
When the TFT is used for, for example, a pixel circuit of a display, the variation in characteristic causes a variation in operation of an organic LED or a liquid crystal element which is to be driven, thereby finally reducing the image quality of the display.